Known methods for preparing p-cumylphenol from .alpha.-methylstyrene and phenol include, for example, the one described in U.S. Pat. No. 2,882,322 which comprises reacting phenol and .alpha.-methylstyrene in the presence of sulfuric acid as a catalyst. However, this method has some disadvantages. For example, it gives only a limited yield or it requires a troublesome operation for the removal of the sulfuric acid catalyst. It is further required to remove light-boiling substances including phenol and heavy-boiling substances including colored materials from the reaction mixture in order to obtain the aimed p-cumylphenol product. When distillation is used for purification processes, it easily causes thermal decomposition of the p-cumylphenol into .alpha.-methylstyrene and phenol during the distillation, even though the acid catalyst was sufficiently removed before distillation. Therefore the obtained product is contaminated with these by-products and it is thus required to purify the same by recrystallization in order to obtain p-cumylphenol of a high purity. This crystalization step makes the procedure complicated and expensive, such that, it is required to recycle a solvent used in this step.
Japanese Patent Publication No. 30614/75 has proposed a process for preparing p-cumylphenol by reacting phenol and .alpha.-methylstyrene in the presence of a sulfonated cation exchange resin catalyst which can be readily removed after the reaction. However this process is accompanied by some troubles such that the ion exchange resin is expensive and that sulfonate ion would be eluted into the reaction mixture. There is merely disclosed in this patent that the aimed p-cumylphenol can be isolated from the reaction mixture by distillation or crystallization, but there is not specifically disclosed therein that the reaction mixture can be distilled to thereby give the aimed p-cumylphenol of a high purity.
Further a process wherein a reaction mixture containing p-cumylphenol is distilled at a low temperature under high vacuum condition to thereby prevent the thermal decomposition of the p-cumylphenol during the distillation is extremely disadvantageous from an industrial viewpoint, since the vapor pressure of p-cumylphenol is low and thus it requires the high degree of vacuum up to 10 mmHg in order to conduct the distillation at a temperature of up to 180.degree. C.